Process for concentrating acetic acid



Jan. 28, 1936.

D. 'F. 'OTHMER PROCESS FOR GONCENTRATING AGE'I'IC Agm 2 Sheets-Sheet '1Filed Feb. 6, 19:1

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ATTORNEY Jan. 28, 1936. D. F. OTHMER 2,028,300.

' rnocass FIOR CONGENTRATING ACETIC ACID Filed Feb. e. 1931 2Sheets-Sheet 2 I INVENTOR DONALD E OTHMER I BYE ATTORNEY Patented Jan.23, 1936 UNITED STATES 2,028,800 rriocsss roa conc grma'rme ACE'I'IO I ADonald F. Othmer, Rochester, N. Y., assignor to Eastman Kodak Company,Rochester, N. Y., a

corporation of New York Application February 1931, Serial No, 513,989

' -This invention relates to an improved process for the concentrationof acetic acid from aqueous solutions thereof, and it is applicable tosolutions of practically any percentage or strength, as well 5 as' tothose which may be contaminated by impurities in solution or suspension.The process of this invention is furthermore applicable to solutions ofacid irrespective of how such solutions .may have been formed orobtained, as for exampie, solutions involving pyroligneous acid, aceticacid obtained as a waste product in acetylation processes such .as. theacetylation of cellulose,

fermentation acid, etc.

Among the numerous methods known for the concentration af acetic acidsolutions, two may be mentioned. One is the method of extracting theacid from its aqueous solution with some ma; terial which is a solventfor the acid, and which isrelatively insoluble in water. Another is the20 method of removing the water or dehydrating the acid by means of adistillation operation employing some added material which may be calleda withdrawing agent, which forms with water on heating, a constantboiling or azeotropic mixture 25 having a boiling point suflicientiy.lower than that of the acetic acid to permit the rectification of themixture and the resulting removal of water from the acetic acid solutionin the usual operations and equipment familiar in the art ofdistillation.

30 It isprimarily with the second or distillation method thatthe presentinvention is concerned, although certain aspects of the invention alsomake use of the features of the first or solvent method. An object ofthe invention is to pro- 35 vide a generally improved, more eilicient,and more satisfactory process for concentrating or dehydrating aceticacid.

Another object is the provision of a process of a simple and efiectivenature capable of employing withdrawing agents from a class heretoforebelieved to be unsuitable.

Still another object is to provide an improved process which results inthe complete rectification of acetic acid without leaving any residue ofthe 5 withdrawing agent therein.

A further object is the provision of a process which willsave asubstantial amount of the heat which it has heretofore been necessary touse in prior processes of concentrating acetic acid.

,0 A still further object is the provision of a process employing awithdrawing agent which has a marked solubility of acetic acid from itssolutions so that the withdrawing agent may also be used as an ex r t na ent for extracting acetic i5 acid from the solution.

Another object is the provision of an improved,

more eflicient, and more satisfactory continuous process for theconcentration of acetic acid.

To these and other ends the invention resides in certain improvementsand combinations of 5 parts, all as will be hereinafter more fullydescribed, the novel features being pointed out in the claims at the endof the specification.

I In the drawings: Fig. 1 is a vapor composition curve of a mixture ofacetic acid and water; Fig. 2 is a vapor composition curve of a mixtureof acetic acid and benzene; Fig. 3 is a vaporcomposition curve of amixture. of acetic acid and propyl acetate;

Fig. 4 is a vapor pressure curve illustrating the vapor pressures ofwater, propylene chloride,

and an azeotropic mixture of the two;

Fig. 5 is a vapor pressure curve illustrating water, benzene, and anazeotropic mixture of the two; 7

Fig. 6 is a similar curve illustrating water, propyl acetate, and anazeotropic mixture of the two, and

Fig. 7 is a diagrammatic view of a preferred 5 form of apparatus forcarrying out the process.

Similar reference numerals throughout the several views indicate thesame parts.

The general use of a withdrawing agent in concentrating or dehydratingan aqueous solution of acetic acid is explained and set forth in BritishPatent No. 327,444, accepted April 3, 1930, which patent is the resultof a prior invention made jointly by Hans T. Clarke and the presentapplicant, Donald F. Othmer. Reference is made to said British patentfor a disclosure of the general principles underlying the use of awithdrawing agent (referred to as an auxiliary liquid in said patent),which need not be repeatedhere. The said patent sets forth eight chiefrequirements for an auxiliary liquid or' withdrawing agent for use indistilling water from aqueous acetic acid, which eight requirementsstill hold in connection with the present invention, with the exceptionof requirement No. 3. This states 45. that the withdrawing agent shouldboil at a lower temperature than acetic acid so that it would be readilyand completely separable from the latter by distillation. The presentinvention contemplates the use of withdrawing agents 50. which dofnotboil sufliciently lower than acetic acid to permit such completeseparation readily by distillation, but this fact is not found to bedetrimental because, according to'the present invention, when theprocess is properly and 'em- :Itdbecome mixed with the concentratedacetic The withdrawing agents which have heretofore been consideredsuitable for use in concentrating acetic acid may be divided in generalinto two classes. The first comprises what might be termed the lowboiling point class, such as ethylene dichloride disclosed in theaforesaid British Pat.- ent No. 327,444, benzol disclosed in UnitedStates Patent No. 1,722,532 to Maude, and ethyl acetate disclosed inBritish Patent No. 28*,588 to I. G. l arbenindustrieAktiengesellschai't. The second group comprises what might be termed thehigh boiling point class such as butyl acetate, as disclosed in BritishPatent No. 298,187 to Dr.

Alexander Wacker Gesellschait.

Withdrawing agents of the low boiling point group have been consideredsuitable because any excess, of withdrawing agents could be readilyseparated from the concentrated acetic acid by distillation. Withdrawingagents of the high boiling point group have been unsuitable from thestandpoint of easy separation oi excesses by distillation, but it hasbeen possible to use them in some circumstances by following the methoddisclosed in said British Patent No. 298,137.

Withdrawing agents boiling between the boiling points of the low boilinggroup and the high boiling group have heretofore been consideredunsuitable and impracticable for use because of the difliculty'oiseparating excesses of such with-.

- drawing agents from the concentrated acetic .ture which will boil at aconstant boiling p t.

. below the boiling point of either water or the.

withdrawing agent alone. Such a constant boilingmixtureis'commonlyknownintheartas an azeotropic mixture" and will behereafter referred to by that designation.

. Most azeotro'pic mixtures having water as one oi the components, suchas the mixtures hereinafter more fully discussed, are found to follow ingeneral the known principles of steam distillation. It is also foundthat if water be mixed with. a liquid which is substantially immiscibletherewith, the respective vapor pressures of'the water and the otherliquid are each substantially unaflecte'd by the presence of the other,and the vapor pressure oi. the is very nearly equal 'to the sum of theindividual vapor pressures of the two liquids. Hence a vapor pressurecurve of the mixture-may be by summing the separate vapor pressurecurves oi the two component liquids.

. it is r una that the molecular composition oi vapors arising fromsuch-mixtures is substantially proportional to the respective vapor "ofthe constituents. The higher thevapor pressure of the other. liquidmixed with water, the greater will be the number 0! molecules oi suchother liquid in any given quantity at vapor evolved i'romthe mixture.

a,o2e,eoo ciently carried out the withdrawing agent does- The efliclencyof a, concentration process involving an azeotropic mixture of water anda withdrawing agent depends to a extent on the relative proportions orwater and withdrawing agent whichare vaporized. A This depends, in turn,upon the relative molecular percentages oi the water and ot thewithdrawing agent, which molecular percentages. as above stated, areapproximately proportional to the respective vaporpressures oi the waterand the withdrawing agent at the particular 'pressure under whichboiling takes place, generally and preferably atmospheric pressure,although subatmospheric orsuperatmospheric pressures may be used ifdesired, Since the normal boiling points oi liquids are temperatures atwhich their vapor pressures are the same, i. e. 76 centimeters ofmercury, it follows that the vapor pressures of diilferent liquids varyin general more or less inversely with their boiling points. Thereforeastatement of the boiling point of a liquid gives a general roughindication of its relative vapor pressure characteristics, the liquidsof higher boiling point having in general-at all temperatures lowervapor pressures than liquids of lower boiling point.

Hence from the standpoint of emciency as determined by thevapor'pressure oi. the withdraw-, ing'agent, it is desirable to usewithdrawing agentshaving relatively high boiling points and consequentlylower vapor pressures. At the same time, the use oi a withdrawing agenthaving a high boiling point is accompanied by the disadvantage that suchan agent will not lowerthe boiling point of an azeotropic mixture withwater -to a sumcient extent to permit easy distillation of theazeotropic mixture .from the acetic acid, the boiling point of which isapproximately 118 C. Alsopa withdrawing agent of high boiling point hasthe further disadvantage that its boilin point is so close to theboiling point oi. acetic acidthat the withdrawing agent is apt tocontaminate the concentrated acid during the dis-- process. Thus inselecting withdrawing agents suitable for use, the advantages or higheror lower boiling points must be balanced with the correspondingdisadvantages, and suitable agents must extent as practicable,theadvantages 01 high boiling point without an impractically largeamount oi the disadvantages thereof.

By balancing the various ifactors in the manneroutlined above, I havediscovered that the most suitable withdrawing agents for use inconcentrating aqueousaoetic acid solutio those which have boiling pointsat normal atmospheric selected which have to as large an pressure oimore than 88 C, and less than 103 C.

Suchwithdrawing agents which have boiling points between 88. 0.. and 103C. have in seneral vapor pressures which, in azeotroplc mixtures at ornear normal atmospheric pressure, are not materially greater than thevapor pressure of the water in the azeotropic mixtures, so that aprocess employing such withdrawing agents has relatively highei'flciency; yet the vapor pressures are sufllciently high so that theboiling point of the azeotropic mixture of the withdrawing agent andwater is suillcientiy lower than the boiling a pointer 'acetic acidto'permit satisfactory-dis tillation'of the electronic mixture from theacetic acid solution and form pure acetic acid.

Viewed from a slightly diflerent aspect, -I find the most suitablewithdrawing agents are those whose vapor pressures, at the-temperatureotthe boilingpolntotiheaaeotropicmixturewithwater, are not more than 60%nor less than 40% of the total vapor pressure of the azeotropic'mixture. When the vapor pressure of the withdrawing agent is more thanabout 60% of the'total vapor pressure of the azeotropic mixture, thenthe molecular percentage of the vapors from the azeotropic mixture issuch that an uneconomically excessive quantity of withdrawing agent mustbe distilled over for each unit quantity of water removed from themixture. Similarly, when the vapor pressure of the withdrawing agentis'less than about 40% of thetotal vapor pressure of the'mixture, thevapor pressure of the withdrawing agent is not sufiiciently high toreduce the boiling point of the 'azeotropic mixture to asuflicientextent to permit easy and satisfactory distillation of theazeotropic mixture from the acetic acid solution and from pure aceticacid. Hence the vapor pressure limits of 60% and 40% above mentioned arefound to be most suitable, and the present invention contemplates theuse of any withdrawing agent having a vapor pressure betweentheselimits.

Viewing the matter from'still another aspect, I find that in general themost suitable withdrawing agents are those which form azeotropicmixtureswith water in which the azeotropic mixture has a normal boiling pointbetween 76 C. and 86C. The three sets of limits above specifledcorrespond in general with each other. That is, withdrawing agents whosevapor pressures are within the specified limits 01' 60% and 40% will befound to have boiling points approximately between the specified rangesof 88 C. and 103 C., and it will further be found, that such agents iormazeotropic mixtures with water which mixtures have boiling pointsapproximately between the specified limits of 76 C. and 86 C.

Thus it is the physical or physical chemical .properties rather than thestrictly chemical properties of the withdrawing agents which are foundto be most important according to the present invention. Of course allliquids which have boiling points between 88 C. and 103 C. would not be.suitable for-the purposes of the present invention, because all oi. suchliquids would-not be proper withdrawing agents. A withdrawing agent, asthe term is understood in the art and .as it is intended in thisspecification and in the accompanying claims, may be defined asaliquidwhich will form a constant boiling or azeotropic mixture withwater. Generally and preferably, a withdrawing agent is substantiallyimmiscible with water, although some withdrawing agents which aremiscible with water arelmown.v Those withdrawing agents which aresubstantially immiscible with water form azeotropic mixtures therewithin which the vapor pressure of the mixture at any temperature issubstantially the sum oi! the separate vapor pressures of water and thewithdrawing agent.

Thus, by definition, the term withdrawing agent as herein usedautomatically excludes all substances which will not form azeotropicmixtures with water. I do ,ilnd, according to the present invention,that'any withdrawing agent,

materials to be more or less advantageous for the process described, theapproximate boiling ,point under normal atmospheric pressure beingindicated after the name of each substance:

Certain of these substances, such as allyl iodide and normal heptane,form azeotropic mixtures at certain percentages with acetic. acid aswell as -with 'water, and consequently have been found useful inconcentrating acetic acid only between certain ranges of strength or inconcentrating acetic acid where an anhydrous acid mixed with someproportion of withdrawing agent may be discharged for use in somefurther operation. The use of awithdrawing agent which forms anazeotropic mixture with acetic acid is not generally desirable, but maybe suitable under special circumstances as above mentioned.Benzene,known in the prior art as a withdrawing agent .(see Maude PatentNo. 1,722,532, above mentioned). forms an azeotropic mixture with aceticacid when themixture contains about 98% benzene, but this does notprevent its use under some circumstances. Similarly the use of awithdrawing agent within the boiling range specified such as normalheptane, which forms an azeotropic mixture with acetic acid, makes itdiflicult or uneconomical to concentrate dilute aqueous acetic acid bythe preferred process described, but if the amount of water in thestarting solution is only comparatively small, the acid may bedischarged trom the still pot in a practically pure condition althoughthe water will also contain substantial amounts of acid.

Other withdrawing agents, which do not form azeotropic mixtures withacetic acid, are in general more suitable than those which do form suchmixtures. It has been found that exceedingly satisfactory results areobtained by using normal propyl acetate, iso-propyl acetate, orpropylene chlcride,-and at present it is preferred to use one or another01' these three substances, which are commonly available at reasonablecost, and es- .pecially the normal propyl acetate.

Other substances mentioned above, as for example diethylene dioxide-(1,4-dioxan), do not, in their. azeotropic mixtures with water; separateinto two layers on standing. Consequently when such a subst ce is usedas a withdrawing agent, it is more cult to separate the agent "from thewater after the distillation, but with-Q drawing agents of this kindmaybe used to advantage under some conditions, and their use is withinthe scope of this-invention.

'To aid in understanding the physical properties of the withdrawingagents and mixtures of the present invention, various curves areillustrated in-Flgs. 1 to 6 inclusive. Referring to these, Fig.

' than acetic acid vapor is given off when a mixture of the two isboiled. This indicates that a simple mixture of water and acetic acidcannot be satis-' factorily concentrated by distillation, and showswhyit is necessary to use a withdrawing agent in order to concentrateaqueous acetic acid coo-'- nomlcally. r

Fig. 2 illustrates a similar vapor composition curve of benzene andacetic acid, in which the percentages indicated refer to benzene. Themain curve is plotted on the main abscissa and the'main ordinatesindicated at the bottom and I left hand side of the graph, respectively,while the supplementary curve is an enlargement of the upper right handend ofthe main curve and is vplotted on the enlarged scale abscissa; andordihates indicated at the top and right hand side of the graph,respectively, as is commonly understood by those familiar with curves ofthis kind. The enlarged supplementary curve clearly shows that thevaporcompositioh curve crosses'the 45 line when the benzene isapproximately 98% of the total mixture, and such crossing of the 45 lineindicates that, at this point an azeotropic mixture of acetic acid andbenzene is formed... Hence, as already mentioned above, the usefulnessof benzene, one of the prior art withdrawing agents, is limited.

The graph of Fig. 3 shows a vapor composition curve of acetic acid andnormal propyl acetate. the percentages of propyl acetate in the mixturebeing indicated by the abscissa; and the percentages of propyl-acetatevapor in the total vapor being shown by the ordinates. This vapor com--position curvev does not cross the 45 line, and

thus indicates that propyl acetate does not form an azeotropic mixturewith acetic acid. Hence propyl acetate'may be satisfactorily used as awithdrawing agent to concentrate aqueous acetic acldof any percentage ofstrength. The close-' ness of the vapor composition curve to the 45 linedoesindicate, however, that it is diflicult to distill an excess ofpropyl acetate from the final concentrated acetic acid in an economicalmanner, and for that reasonthe concentration is, preferably carried onby a continuous process hereinafter described which does not result indicated by the line I i, and a-vapor pressure curve of an azeotropicmixture of water and propylene vchlqflde indicated by the line l2. Thetotal vapor pressure of the mixture at any particular temperature issubstantially the sum of thetwo separate vapor pressures at thattemperature, as explained above. In this graph, the abscissa: are

graduated in degrees centigrade from 50 to 100,

as the portions of the curves below 50 C. are unnecessary for thepresent purposes. The ordinates are graduated -in pressures expressed ascentimeters of mercury from O to 80. The reference line I! indicates'apressure of 76 centimeters or normal atmospheric pressure.

It will be seen that the curve I! of the mix,-

ture crosses the line ll of normal atmospheric pressure at approximately78- 0., indicating this 5 as the boiling point of the mixture, which isbe tween the limits of 76 C. and 86 C. above mentioned. Of the totalvapor pressure of '16 centimeters. of mercury, about 33 centimeters isdue to the vapor pressure of the water, as indicated by the bracket I 4,while the remainder or about 43 centimeters is due to the vapor pressureof the propylene chloride, as indicated" by either of the two bracketswhich subtend an equal distance. Hence it will be seen that the vaporpressure of the propylene chloride at the boiling point of the is about57% of the. total vapor pressure of the mixture, and is between thelimits of 60% and 40% above mentioned.

As the vapor pressures of the components of a mixture of this kind arevery nearly proportionate to the respective molecular percentages, itfollows that of every '76 molecules in the vapor evolved from the liquidmixture, about 33 molecules will be water molecules and about '43molecules will be propylene chloride molecules. By reducing topercentages 'and' multiplying the molecular percentage by the molecularweights, the relative quantities by weight of water and of propylenechloride evolved'from' boiling an azeotropic mixture of the two can bereadily calculated. when this is dbt'i'ey'it is found that of pylenechloride. Multiplying this by the latent heat of vaporization ofpropylene chloride, which is '75 calories per gram, it is found that 600calories are necessary to distill the quantity of withdrawing agentneeded to carry over each gram or water in the mixture. This irelatively eflicient in comparison to withdrawing agents which have beenused in the prior art, in which much greater proportions of withdrawingagents have ordinarily had to be distilled, with consequent greaterconsumption of heat. For the sake of comparison, Fig. 5 shows a similarset of vapor pressure curves of water and benzene, one of the prior artwithdrawing agents,

and of a mixture of the two. The curve 20 is that of water, II that ofbenzene, and 22 that of the azeotropic mixture of the two. Line 23indicates normal atmospheric pressure of 16 centlmeters, and crosses theline 22 at about 68, indicating this as the boiling point of themixture. The bracket 24 indicates that of the total vapor pressure of'76 centimeters only a relatively small proportion or about 22centimeters is due' to the vapor pressure of the water, while arelatively large proportion or about I4 centimeters is due to the. vaporpressure of the benzene as indicate'd by the brackets 25. Hence whenusing benzene, according to the prior art, it is'necessary to distillmany more molecules of the with- 'drawing agent for each given quantityof water 7 molecules distilled than when using propylene chlorideaccording to the present, invention; It" will be seen that the vaporpressure and consequently the molecular percentage of the benzene ismorethan-twice that of the water, and about 76 71% of the total vaporpressure of the mixture.- When multiplying the molecular percentages bythe molecular weights as above indicated, it is found that for everyunit of water distilled from an azeotropic mixture of water and benzene,it is to distill 10.6 units by weight of henxene, and since the latentheat of vaporization of benzene-is 94, it requiresalmost exactly 1000calorles to distill the withdrawing agent necessary cording to thepresent invention.

to carry over one gram of water from the mixture.- This will be seen tobe much more wasteful of heat than when propylene chloride is used ac-Referring now to Fig. 6, there are shown corre-.

sponding vapor pressure curves of water, in-' dicafed by' the numeralll, normal propyl acetate, indicated by the numeral II, and anazeotropic mixture of .water and propyl acetate, indicated at 12. Thereference line 33 indicates normal atmospheric pressure of 76centimeters, and shows that the boiling point of an azeotropic mixtureat normal pressure is about 81 C..which is suillcientlyfar removed fromthe boiling point of acetic acid (about 118 C.) to melt possibletodistill the azeotropic mixture from the acetic acid in Fig. 5.

Multiplying the-molecular percentages by the molecular weights, itisfound that of a unitof weight of vapor, about 14.4% by weight is watervapor and about 85.6 by weight is propyl acetate vapor, or in otherwords, 5.9 units by weight of propyl acetate must be distilled'with eachunit of weight of water. Multiplying this by the latent heat ofvaporization of propyl acetate, it is found that 472 calories must beused to distill the amount of withdrawing agent necessary to carry overeach gram of water from the azeotropic mixture. This is a great dealmore eflicient than when benzene is used as the withdrawing agent, andsomewhat more efllcient than when propylene chloride is used. Hence Iprefer when practicing the present invention to employ normal propylacetate, although I also contemplate using any other withdrawing agentboiling within the temperaturerange of 88 'C. to 103- C., as abovementioned, and especially'iso-pro'pyl acetate or propylene chloride.

The boiling points, molecular percentages, and

. other figures given above by way of example are those obtained bycalculation. In actual practice the results are found to be slightlydifferent from but quite close to those calculated, and in general it isfound that the actual results are to a slight extent even moreadvantageous and efficient thaiithecalculated ones.

The various temperatures and vapor pressures above mentioned have beendetermined on the assumption that distillation will take place at normalatmospheric pressure of76 centimeters of mercury, since this isordinarily prefen'ed. It is possible, however, to carry out the processof the present invention at pressures either above or ciencies withrespect to withdrawing agents of the i prior art. I

The use of normal propyl acetate as a withdrawing agent has a furtheradvantage not only because of its increased emciency over withdrawingagents of the prior art, but also because it has a substantialsolubility for acetic while being substantially insoluble in water.Hence propyl acetate may bev used also as an extracting agent inaddition to its action as a withdrawing want. For instance, the propylacetate may be brought into, contact with the dilute acetic acid todissolve as much as possible of the acetic acid in the propyl acetate,and then the propyl acetate with 7 acid dissolved therein be distilled.

Referring now to Fig. 7 of the drawings, there is shown a diagrammaticrepresentation of a preferred embodiment of apparatus for carrying outthe present invention. The numeral 40 inmaybeledtothestillto dicates acolumn still of anysuitable and well known construction. For example,the still may be of the type having a multiplicity of plates and theusual bubble caps. It is heated in any suitable manner, such as by thecoil 4| to which steam or other heating fluid is supplied from the inlet42 and discharged from theoutlet 43.

To aid in controlling the process, the still is provided at intervalsthroughout its-height with suitable temperature indicating devices suchas the thermometers 44. I

To start the process the still is initially charged with the aqueousacetic acid which is to be concentrated, and with the withdrawing agentse-' lected for use, supplied through the conduit 45 from the container46. Ordinarily, after the still is initially charged with withdrawingagent, the

valve in the conduit 45 is closed and no further withdrawing agent isintroduced from the container 46 except as small amounts may be requiredfrom time to time to make up for losses in the system by leakage. Thewithdrawing agent with which the still is originally charged flowsthrough a continuous circuit and is returned to the still for use overand over again, as described hereinafter.

In the manner above described, the 'withdrawing agent will form anazeotropic mixture with the water in the aqueous acetic acid and theboiling point of this azeotropic mixture will be willciently lower thanthe boiling point of acetic acid so that the azeotropic mixture will bevaporized and will pass off from the top of the still through theconduit 50 and into the condenser 5| where it is condensed and whence itissues from the outlet 52 and flows to the gravity or other suitableseparator' 53; The acetic acid, meanwhile, gradually works downwardly inthe column and may be removed from the bottom thereof in liquid phase ifdesired, although it is preferred to remove the concentrated acetic acidin vapor phase through the outlet 54 leading to the condenser 55, whereit is condensed to liquid phase and discharged through the conduit ,56to any desired point. i

The process is' preferably carried on continuously once the still hasbeen charged and started, additional aqueous acet acid being supplied tothe still through the .ilet conduit Bit leading from any suitable sourceof supply such as the reservoir iii. The aqueous acetic acid suppliedthrough the conduit 60 may be in either liquid or vapor phase, and theconduit may enter the column at an intermediate point in theheightthereof, which pointwill be determined in the usual manner wellunderstood by those skilled in the art.

The azeotropic mixture of water and withdrawing agent, after enteringthe separator 53,

separates into two layers, a lighter layer at the top and a heavierlayer at the bottom. A con- -duit 64 leads from the layer of withdrawingagent to the still so that the withdrawing agent after being separatedis returned substantially continuously to the still to be used over andover again. Incase the withdrawing agent employed is propyl acetate,which is lighter than water, the top layer 65 in the separator 53 willbe propyl acetate while the bottom layer 65 will bewater. Consequentlythe conduit 64 is connected'to the the withdrawing agent from the water,there is some slight amount of withdrawing agent' Frequently the amountdissolved in the water. of withdrawing agent dissolved in the water-isnot suflicient to justify recovery thereof. and the conduit 61may-then-lead to waste. but if it is desired under any particulareconomic conditions 'to recover the dissolved withdrawing agent, then rtheconduit 6T may be led tov any suitable recovery apparatus, such forexample as the auxiliary still 58 supplied with steam through the inlet69.

In this still, the dissolved withdrawing agent is separated from thewater and the vapors of the withdrawing. agent pass ofl from the top ofthe still through the conduit 10*to a condenser II in which they arecondensed and from which they are led through the conduit 12 to theseparator 53. Thus the withdrawing agent dissolved in the water layer ofthe separator is recovered and returned to the separator, whence it,together with the withdrawing agentflrst separated from the water in theseparator, is supplied to the main still II for further use. The waterseparated from the withdrawing agent in the auxiliary still 68 may bedischarged therefrom through the discharge conduit ll.

Since substantially all of vthe withdrawing agent distilled off from thecolumn still through the conduit 50 returns to the column through theconduit 84, it follows that the-rate of supply of the withdraw a' 8nt tothecolumn is ordinarily not variable, when the proper amount is in thesystem. The variable factors of the process are the rate at which heatis supplied through the conduit 42 and the rate at which acetic' acid issupplied through the conduit 80. These two rates are varied desired inorder to'obtain proper operation of the apparatus.

It isordinarily desirable in operation to have at all points in thecolumn the correct aseotropic ratio of withdrawing agent to the water inthe acetic acid, even though it is only at'the top of the columnthat thevapors are composed wholly of azeotropic mixture to the exclusion ofacetic acid vapor. At successively lower points in the column, whereacetic acid is present in relatively greater and greater amounts andwater a branch of the conduit 64 may lead to some lower 10 point of thecolumn at which a small fraction of the returning withdrawing agent maybe supplied. Heretofore, prior processes of concentrating acetic acidhave usually employed ex of withdrawing agent, rather than just theright 15 amount of withdrawing agent as contemplated by the presentinvention. Hence withdrawing agent v has heretofore been present at mostpoints of the column, the lower portion of'the column hav ing nowaterpresent but only a mixture of acetic acidand withdrawing agent. Thepresent invention contemplates that the variable factors (rate ofheating and rate of supply of aqueous acetic acid) should be socontrolled that there will be no excess of withdrawing agent, and thatat the point in the column at'which the last of the water disappears,the last of the withdrawing agentwill also disappear.

By watching the thermometers or other heat indicating devices 44 at thevarious points up and 30 down the column, an operator skilled in theoperation of a column still is enabled, to ascertain very closely thecomposition ofthe mixture at various points in the column, and in thisway he is enabled to control the process to accomplish :5 the desirableresult above set forth, varying the rate of heating and acetic acid suply as necessary, and adding additional withdrawing agen from timetotime if this is found to be n--- When the process is carriedon in theabove I desired manner according to the present invention', the stillpot will contain substantially pure or glacial acetic acid. instead ofacid having withdrawing agent mixed therewith as' has usualLv .been' thecase hemtotore. Consequently there 45 acid is substantially less thanwould be the caseif less eflicient withdrawing agents such asthose ofthe prior art were employed. Hence the varic9 ous parts of .theapparatus can be made smaller .thanwouldotherwisebethe case, sincealessquantity of material need be handled. and this results in a substantialsaving in cost as apparatus 'of this character is generally expensivebecause of its. :1 chemical qualities.

According to a modifled process contemplated by the present invention,instead of adding a withdrawing agent to the dilute acetic acid, the

agent may be formed by chemical t reaction in the dilute acetic aciditself, the sub-- stanceadded reacting either'with the acetic acid. orwith the water which'dilute's it. For example, propyl alcohol may beadded to acidtombyreaction therewimmlaeo- 'in the manner previouslydescribed,

tate, which is the perm withdrawing agent 01' the present invention.Hence the present invention is intended to include withdrawing agentsformed or added in this manner, provided' they fall within thetemperature and other ranges specified, as well as those formedexternally and added later to the dilute acetic acid. The presentinvention also contemplates the use. as a withdrawing agent of amixture01' any two or more substances which when mixed will ,boil'or have othercharacteristics within the ranges heretofore specified, regardless ofwhat the boiling points or other characteristics of the individualcomponents bei'ore mixture may be According to another embodiment oi theprocess, instead of passing the withdrawing agent directly from theseparator 53 to the still column 40, through the conduit 84, sometraction of it may first be led to an extractor of any suitable knowntype for extracting one liquid by another, in

which extractor the withdrawing agent will come into contact withaqueous acetic acid and extract the acetic acid from the water. Themixture of withdrawing agent, acetic acid, and any small mixed with theacetic acid is discharged from the extractor saturated with thewithdrawing agent, and it may then be introduced through the conduit 61into the-auxiliary still 68 to have the withdrawing agent recovered inthe manner previously described, the water free from with-s drawingagent being discharged through the conduit 13.. p

While certainembodiments of the invention have been disclosed, it is tobe understood that the inventive idea may be carried out in a number ofways. This application is therefore not to be limited to the precisedetails described, but is intended to cover all variations andmodifications thereof falling within the spirit of the invention or thescope or the appended claims;

I claim:

, 1. The continuous process of concentrating aqueous acetic acid toproduce a concentrated acid free of withdrawing agent which comprisescharging a still with a mixture of aqueous acetic acid and just theright amount or a water-withdrawing agent, selected irom the groupconsisting of normal-propyl acetate, iso-propyl acetate and propylenechloride, to form an azeotropic mixture with the water in the aqueousacetic acid, distilling water and withdrawing agent ironithe mixture,condensing the distillate and substantially separating the withdrawingagent from the water therein, returning the separated group consistingof normal-propyl acetate, isopropyl acetate and propylene chloride, toform an c mixture. with the water in the increased I aqueous aceticacid,-distilling water and withdrawing agent from the'mixture,condensing the distillate and substantially separating the with drawingagent from the water therein, returning the separated withdrawing agentsubstantially continuously. to the still and supp ying further aqueousacetic acid substantially continuously to the still, the withdrawingagent being returned to the head of the column still at a ratesuiiicient to form an azeotropic mixture with the major proportion ofthe water contained in the aqueous acetic acid being supplied to thestill and to the column still at apoint somewhat belowthe head c1 thecoiumn at a'rateapproximately to form an azeotropic mixture with theremaining water contained in the aqueous acetic plied to the still.

3. The continuous process of concentrating .acetate from the watertherein, returning the separated propyl acetate substantiallycontinuously to the still and supplying further aqueous acetic acidsubstantially continuously to the still, the propyl acetate beingreturned to the still at a rate just right to form an azeotropic mixturewith the water contained in the aqueous acetic acid being suppliedto'the still. 1

4. The continuous process of concentrating g aqueous acetic acid toproduce a concentrated acid free of withdrawing agent which comprisescharging a column still with a mixture of aqueous acetic acid andapproximately the right amount of propyl acetate to form an azeotropictially continuously to the still, the propyl acetate being returned tothe head of the column still a at a rate sufiicient to form anazeotropic, mixture with the major proportion of the water contained inthe aqueous acetic acid being supplied to the still and to the columnstill at a point somewhat below the head of the column at 'a rateapproximately sufficient to form an azeotropic mixture with theremaining water containedin the aqueous acetic acid being supplied tothe still.

5. Thecontinuous process of concentrating aqueous acetic acid to producea concentrated acid free of propylene chloride which comprises charginga still with a mixture of aqueous acetic. acid and just the rightalnount of propylene chloride to form an azeotropic mixture with thewater in the aqueous acetic acid,- distilling water and propylenechloride from the mixture, condensing the distillate and substantiallyseparight to i'orman 'azeotropic mixture with the 15 acid being supiwater contained inthe acetic acid being 6. The continuous process ofconcentrating aqueous acetic acid to produce a concentrated acid free ofpropylene chloride which comprises chargingacolumnstillwithamixtureofaqueous acetic acid and approximately the right amount of propylenechloride to form an aaeotropic mixture with the water in the aqueousacetic acid, distillingwater and propylene chloride from themixture,condensing the distillate and substantially separating the propylenechloride from the water therein, returning the sepa-, rated propylenechloride substantially continuously to the still and supplying furtheraqueous acetic acid substantially continuously to the: still,

the propylene chloride being returned to the head of the column still ata rate sufllcient to form an aaeotropic mixture with the majorproportion or the water contained inithe aqueous acetic acid beingsupplied to the still and to the column still at a point somewhat belowthe head of column at a rate approximately sufllcient toform -anaaeotropic mixture with the .distillin'g' water and withdrawing agentfrom the mixture, condensing the distillate and substantially separatingthe withdrawing agent from the remaining water contained; in the aqueousacetic a,oas,ao'o A wa'ter therein, returning the separated withdrawingagent substantially continuously to the still, supplying iurther aqueousacetic acid substantially continuously to the still, the withdrawringagent being returned to the still at a rate Just right to form anazeotropic mixture with the water contained in 'the aqueous acetic acidbeing supplied to the still, and further distilling water andwithdrawing agent from-the mixture, thereby leaving a residue containingconcentrated acetic acid, heating said residue sufllciently to cause atleast some of the concentrated acid to exist in the vapor phase,conducting away these acetic acid vapors free of withdrawing agent andcondensing these vapors to theliquid phase.

8. The continuous process of concentrating aqueous acetic acid byazeotropic distillation to produce an acid free of propyl acetate whichcomprises supplying a column still with aqueous acetic acid and reactingtherewith, in a manner which will produce propyl acetate; an amount oipropyl aclohol sufllcient to produce in said acid a quantity of propylacetate in Just the right amount to form azrazeotropic mixture with thewater in the aqueous acetic acid to be concentrated, distilling waterand pr'opyl acetate from the mixture, condensing'the distillate andsub-,

stantlally separating the propyl acetate from the water therein,returning the-separated propyl and suWMDE further' aqueous acetic acidsubstantially continuously to the still, the propyl acetatesubstantially continuously to the still,

acetate being returned to the still at a rate just right to iorm anazeotropic mixture with the water contained in the aqueous acetic acidbeing supplied to'the DONALD F. OTHIMER.

terri c-ATE or coarser-Ion;

( Seal s Patent No. 2,028,800. January 28-1936.

Dorm-o r. I

It Els'herebylcert iiied that error-appears in the pri' ntedspecivfication' 'of the above .numbe'r'ed-.-patent requiring correctionas" follows: Page 7, second;

column, .line'14, clainh -after "approximately" insert the wordsufficient, and that the said Letters Patent should be read withxthiscorrection therein;

that the same may conform toothe record. or the case in-the Patent0fiice'.-.i

Signed and sealed this 5th'day-llay, A D. 1936:.1' 7' Leslie FrazerActing Commissioner: of Patents water contained inthe acetic acid being6. The continuous process of concentrating aqueous acetic acid toproduce a concentrated acid free of propylene chloride which compriseschargingacolumnstillwithamixtureof aqueous acetic acid and approximatelythe right amount of propylene chloride to form an aaeotropic mixturewith the water in the aqueous acetic acid, distillingwater and propylenechloride from themixture, condensing the distillate and substantiallyseparating the propylene chloride from the water therein, returning thesepa-, rated propylene chloride substantially continuously to the stilland supplying further aqueous acetic acid substantially continuously tothe: still,

the propylene chloride being returned to the head of the column still ata rate sufllcient to form an aaeotropic mixture with the majorproportion or the water contained inithe aqueous acetic acid beingsupplied to the still and to the column still at a point somewhat belowthe head of column at a rate approximately sufllcient toform -anaaeotropic mixture with the .distillin'g' water and withdrawing agentfrom the mixture, condensing the distillate and substantially separatingthe withdrawing agent from the remaining water contained; in the aqueousacetic a,oas,ao'o A wa'ter therein, returning the separated withdrawingagent substantially continuously to the still, supplying iurther aqueousacetic acid substantially continuously to the still, the withdrawringagent being returned to the still at a rate Just right to form anazeotropic mixture with the water contained in 'the aqueous acetic acidbeing supplied to the still, and further distilling water andwithdrawing agent from-the mixture, thereby leaving a residue containingconcentrated acetic acid, heating said residue sufllciently to cause atleast some of the concentrated acid to exist in the vapor phase,conducting away these acetic acid vapors free of withdrawing agent andcondensing these vapors to theliquid phase.

8. The continuous process of concentrating aqueous acetic acid byazeotropic distillation to produce an acid free of propyl acetate whichcomprises supplying a column still with aqueous acetic acid and reactingtherewith, in a manner which will produce propyl acetate; an amount oipropyl aclohol sufllcient to produce in said acid a quantity of propylacetate in Just the right amount to form azrazeotropic mixture with thewater in the aqueous acetic acid to be concentrated, distilling waterand pr'opyl acetate from the mixture, condensing'the distillate andsub-,

stantlally separating the propyl acetate from the water therein,returning the-separated propyl and suWMDE further' aqueous acetic acidsubstantially continuously to the still, the propyl acetatesubstantially continuously to the still,

acetate being returned to the still at a rate just right to iorm anazeotropic mixture with the water contained in the aqueous acetic acidbeing supplied to'the DONALD F. OTHIMER.

terri c-ATE or coarser-Ion;

( Seal s Patent No. 2,028,800. January 28-1936.

Dorm-o r. I

It Els'herebylcert iiied that error-appears in the pri' ntedspecivfication' 'of the above .numbe'r'ed-.-patent requiring correctionas" follows: Page 7, second;

column, .line'14, clainh -after "approximately" insert the wordsufficient, and that the said Letters Patent should be read withxthiscorrection therein;

that the same may conform toothe record. or the case in-the Patent0fiice'.-.i

Signed and sealed this 5th'day-llay, A D. 1936:.1' 7' Leslie FrazerActing Commissioner: of Patents

